Lipid rafts control P2X3 receptor distribution and function in trigeminal sensory neurons of a transgenic migraine mouse model

Lipid raft disruption as an opportunity for peripheral analgesia

Chronic pain conditions are unmet medical needs, since the available drugs, opioids, non-steroidal anti-inflammatory/analgesic drugs and adjuvant analgesics do not provide satisfactory therapeutic effect in a great proportion of patients. Therefore, there is an urgent need to find novel targets and novel therapeutic approaches that differ from classical pharmacological receptor antagonism. Most ion channels and receptors involved in pain sensation and processing such as Transient Receptor Potential ion channels, opioid receptors, P2X purinoreceptors and neurokinin 1 receptor are located in the lipid raft regions of the plasma membrane. Targeting the membrane lipid composition and structure by sphingolipid or cholesterol depletion might open future perspectives for the therapy of chronic inflammatory, neuropathic or cancer pain, most importantly acting at the periphery.

Role of ATP in migraine mechanisms: focus on P2X3 receptors

Migraine is a major health burden worldwide with complex pathophysiology and multifarious underlying mechanisms. One poorly understood issue concerns the early steps in the generation of migraine pain. To elucidate the basic process of migraine pain further, it seems useful to consider key molecular players that may operate synergistically to evoke headache. While the neuropeptide CGRP is an important contributor, we propose that extracellular ATP (that generally plays a powerful nociceptive role) is also a major component of migraine headache, acting in concert with CGRP to stimulate trigeminal nociceptive neurons. The aim of the present focused review is to highlight the role of ATP activating its P2X3 membrane receptors selectively expressed by sensory neurons including their nerve fiber terminals in the meninges. Specifically, we present data on the homeostasis of ATP and related purines in the trigeminovascular system and in the CNS; the basic properties of ATP signalling at peripheral and central nerve terminals; the characteristics of P2X3 and related receptors in trigeminal neurons; the critical speed and persistence of P2X3 receptor activity; their cohabitation at the so-called meningeal neuro-immune synapse; the identity of certain endogenous agents cooperating with ATP to induce neuronal sensitization in the trigeminal sensory system; the role of P2X3 receptors in familial type migraine; the current state of P2X3 receptor antagonists and their pharmacological perspectives in migraine. It is proposed that the unique kinetic properties of P2X3 receptors activated by ATP offer an interesting translational value to stimulate future studies for innovative treatments of migraine pain.

Transient receptor potential vanilloid subtype 1 depletion mediates mechanical allodynia through cellular signal alterations in small-fiber neuropathy

Transient receptor potential vanilloid subtype 1 (TRPV1) is a polymodal nociceptor that monitors noxious thermal sensations. Few studies have addressed the role of TRPV1 in mechanical allodynia in small-fiber neuropathy (SFN) caused by sensory nerve damage. Accordingly, this article reviews the putative mechanisms of TRPV1 depletion that mediates mechanical allodynia in SFN. The intraepidermal nerve fibers (IENFs) degeneration and sensory neuronal injury are the primary characteristics of SFN. Intraepidermal nerve fibers are mainly C-polymodal nociceptors and Aδ-fibers, which mediated allodynic pain after neuronal sensitization. TRPV1 depletion by highly potent neurotoxins induces the upregulation of activating transcription factor 3 and IENFs degeneration which mimics SFN. TRPV1 is predominately expressed by the peptidergic than nonpeptidergic nociceptors, and these neurochemical discrepancies provided the basis of the distinct pathways of thermal analgesia and mechanical allodynia. The depletion of peptidergic nociceptors and their IENFs cause thermal analgesia and sensitized nonpeptidergic nociceptors respond to mechanical allodynia. These distinct pathways of noxious stimuli suggested determined by the neurochemical-dependent neurotrophin cognate receptors such as TrkA and Ret receptors. The neurogenic inflammation after TRPV1 depletion also sensitized Ret receptors which results in mechanical allodynia. The activation of spinal TRPV1(+) neurons may contribute to mechanical allodynia. Also, an imbalance in adenosinergic analgesic signaling in sensory neurons such as the downregulation of prostatic acid phosphatase and adenosine A1 receptors, which colocalized with TRPV1 as a membrane microdomain also correlated with the development of mechanical allodynia. Collectively, TRPV1 depletion-induced mechanical allodynia involves a complicated cascade of cellular signaling alterations.

Pharmacokinetic and Lipidomic Assessment of the In Vivo Effects of Parishin A-Isorhynchophylline in Rat Migraine Models

Migraine is a chronic brain disease that leads to periodic neurological attacks. Parishin A and isorhynchophylline (PI) is the active monomer component extracted from the traditional antimigraine Chinese medicinal combination of Gastrodia and Uncaria, respectively. In this study, using high-performance liquid chromatography coupled with tandem mass spectrometry (HPLC-MS/MS) technology, we performed pharmacokinetic and lipidomic study on migraine model rats after administration of PI. For the detection of the compounds in plasma, AB Sciex Triple Quad™ 4500 was applied for quantitative analysis, and the COSMOSIL C₁₈ column (2.1 × 100 mm, 2.6 μm) was used for separation. Isorhynchophylline (ISO: m/z 384.8-241.2) and its main metabolite rhynchophylline (RHY: m/z 384.8-160.2) were simultaneously detected under positive ion modes. Besides, parishin A (PA: m/z 995.1-726.9) and its main metabolite gastrodin (GAS: m/z 331.1-123.0) were simultaneously detected with negative ion modes. For the analysis of endogenous lipid components, Dionex Ultimate 3000 (UHPLC) Thermo Orbitrap Elite was applied for the detection, and the Waters UPLCRBEH C₁₈ column (1.7 μm 100 ∗ 2.1 mm) was used for separation. Chloroform/methanol (2 : 1, v : v) was used for extraction. The results demonstrated that PI exists significant difference in metabolism between single- and coadministration and can regulate lipid levels associated with migraine.

P2X receptors and trigeminal neuralgia

There is currently no effective cure for trigeminal neuralgia (TN) - a relatively common disease that causes long-term pain in patients. Previous research has shown that ionotropic ATP signaling through excitatory and calcium-permeable P2X receptor channels plays a critical role in pathological pain generation and maintenance. In this paper, we review several hypotheses on the pathogenic mechanisms underlying TN. We further discuss pathways or agents that can target P2X expression in TN, thereby affecting pain induction and maintenance.

Lipid rafts in glial cells: role in neuroinflammation and pain processing

Activation of microglia and astrocytes secondary to inflammatory processes contributes to the development and perpetuation of pain with a neuropathic phenotype. This pain state presents as a chronic debilitating condition and affects a large population of patients with conditions like rheumatoid arthritis and diabetes, or after surgery, trauma, or chemotherapy. Here, we review the regulation of lipid rafts in glial cells and the role they play as a key component of neuroinflammatory sensitization of central pain signaling pathways. In this context, we introduce the concept of an inflammaraft (i-raft), enlarged lipid rafts harboring activated receptors and adaptor molecules and serving as an organizing platform to initiate inflammatory signaling and the cellular response. Characteristics of the inflammaraft include increased relative abundance of lipid rafts in inflammatory cells, increased content of cholesterol per raft, and increased levels of inflammatory receptors, such as toll-like receptor (TLR)4, adaptor molecules, ion channels, and enzymes in lipid rafts. This inflammaraft motif serves an important role in the membrane assembly of protein complexes, for example, TLR4 dimerization. Operating within this framework, we demonstrate the involvement of inflammatory receptors, redox molecules, and ion channels in the inflammaraft formation and the regulation of cholesterol and sphingolipid metabolism in the inflammaraft maintenance and disruption. Strategies for targeting inflammarafts, without affecting the integrity of lipid rafts in noninflammatory cells, may lead to developing novel therapies for neuropathic pain states and other neuroinflammatory conditions.

P2X3 receptors are transducers of sensory signals

Peripheral stimuli are transduced by specific receptors expressed by sensory neurons and are further processed in the dorsal horn of spinal cord before to be transmitted to the brain. While relative few receptor subtypes mediate the initial depolarisation of sensory neurons, an impressive number of molecules and ion channels integrate these inputs into coded signals. Soluble mediators and ambient conditions further shape these processes, potentially triggering peripheral and central sensitisation, or sensory downregulation. Extracellular ATP is a major signaling molecule that acts via purinergic receptors and is a powerful modulator of cell communication as well as a neurotransmitter at peripheral/central synapses. In particular, ATP-mediated signals are transduced by P2X3 receptors expressed mainly by peripheral sensory neurons. Recent evidence suggests that P2X3 receptor function not only induces neuron depolarisation and firing with consequent neurotransmitter release, but it also triggers intracellular molecular changes that amplify purinergic signaling with important consequences.

Treatment with methyl-β-cyclodextrin prevents mechanical allodynia in resiniferatoxin neuropathy in a mouse model

Specialized microdomains which have cholesterol-rich membrane regions contain transient receptor potential vanilloid subtype 1 (TRPV1) are involved in pain development. Our previous studies have demonstrated that the depletion of prostatic acid phosphatase (PAP) – a membrane-bound ectonucleotidase ­– and disordered adenosine signaling reduce the antinociceptive effect. The role of membrane integrity in the PAP-mediated antinociceptive effect in small-fiber neuropathy remains unclear, especially with respect to whether TRPV1 and PAP are colocalized in the same microdomain which is responsible for PAP-mediated antinociception. Immunohistochemistry was conducted on the dorsal root ganglion to identify the membrane compositions, and pharmacological interventions were conducted using methyl-β-cyclodextrin (MβC) – a membrane integrity disruptor that works by depleting cholesterol – in pure small-fiber neuropathy with resiniferatoxin (RTX). Immunohistochemical evidence indicated that TRPV1 and PAP were highly colocalized with flotillin 1 (66.7%±9.7%) and flotillin 2 (73.7%±6.0%), which reside in part in the microdomain. MβC mildly depleted PAP, which maintained the ability to hydrolyze phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and delayed the development of mechanical allodynia. MβC treatment had no role in thermal transduction and neuronal injury following RTX neuropathy. In summary, this study demonstrated the following: (1) membrane cholesterol depletion preserves PAP-mediated antinociception through PI(4,5)P2 hydrolysis and (2) pain hypersensitivity that develops after TRPV1(+) neuron depletion-mediated neurodegeneration following RTX neuropathy is attributable to the downregulation of PAP analgesic signaling.

Summary: The role and mechanism of cholesterol-rich membrane integrity in pain development for small-fiber neuropathy remains unclear. Depletion of membrane cholesterol contents preserves functional PAP profiles and the antinociceptive effect after RTX neuropathy.

ATP-Gated P2X3 Receptors Are Specialised Sensors of the Extracellular Environment

P2X3 receptors are ion channels expressed by autonomic and sensory nerves and specialised in transducing extracellular ATP signals. Structural data, together with functional and biochemical studies, suggest that conformational changes of P2X3 receptors upon agonist binding influence downstream intracellular molecular mechanisms relevant for neuronal responses. Activity of P2X3 receptors is implicated in pain, itch, asthma, cardiovascular dysfunction and other pathologies. The study of these receptors has therefore a large potential in the field of drug development and interdisciplinary efforts could clarify molecular mechanisms controlling P2X3 receptor function in different physiological or pathological contexts.

P2X receptor channels in chronic pain pathways

Chronic pain is a highly prevalent debilitating condition for which treatment options remain limited for many patients. Ionotropic ATP signalling through excitatory and calcium-permeable P2X receptor channels is now rightfully considered as a critical player in pathological pain generation and maintenance; therefore, their selective targeting represents a therapeutic opportunity with promising yet untapped potential. Recent advances in the structural, functional and pharmacological characterization of rodent and human ATP-gated P2X receptor channels have shed brighter light on the role of specific subtypes in the pathophysiology of chronic inflammatory, neuropathic or cancer pain. Here, we will review the contribution of P2X3, P2X4 and P2X7 receptors to chronic pain and discuss the opportunities and challenges associated with the pharmacological manipulation of their function.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Membrane cholesterol depletion as a trigger of Nav1.9 channel-mediated inflammatory pain

Cholesterol is a major lipid component of the mammalian plasma membrane. While much is known about its metabolism, its transport, and its role in atherosclerotic vascular disease, less is known about its role in neuronal pathophysiology. This study reveals an unexpected function of cholesterol in controlling pain transmission. We show that inflammation lowers cholesterol content in skin tissue and sensory DRG culture. Pharmacological depletion of cellular cholesterol entails sensitization of nociceptive neurons and promotes mechanical and thermal hyperalgesia through the activation of voltage-gated Nav1.9 channels. Inflammatory mediators enhance the production of reactive oxygen species and induce partitioning of Nav1.9 channels from cholesterol-rich lipid rafts to cholesterol-poor non-raft regions of the membrane. Low-cholesterol environment enhances voltage-dependent activation of Nav1.9 channels leading to enhanced neuronal excitability, whereas cholesterol replenishment reversed these effects. Consistently, we show that transcutaneous delivery of cholesterol alleviates hypersensitivity in animal models of acute and chronic inflammatory pain. In conclusion, our data establish that membrane cholesterol is a modulator of pain transmission and shed a new light on the relationship between cholesterol homeostasis, inflammation, and pain.

Accessibility of axonal G protein coupled mu-opioid receptors requires conceptual changes of axonal membrane targeting for pain modulation

The mechanisms of axonal trafficking and membrane targeting are well established for sodium channels, which are the principle targets for perineurally applied local anaesthetics. However, they have not been thoroughly investigated for G protein coupled receptors such as mu-opioid receptors (MOR). Focusing on these axonal mechanisms, we found that axonal MOR functionality is quite distinct in two different pain states, i.e. hindpaw inflammation and nerve injury. We observed axonal membrane MOR binding and functional G protein coupling exclusively at sites of CCI nerve injury. Moreover at these axonal membrane sites, MOR exhibited extensive co-localization with the membrane proteins SNAP and Na/K-ATPase as well as NGF-dependent enhanced lipid rafts and L1CAM anchoring proteins. Silencing endogenous L1CAM with intrathecal L1CAM specific siRNA, disrupting lipid rafts with the perineurial cholesterol-sequestering agent MβCD, as well as suppressing NGF receptor activation with the perineurial NGF receptor inhibitor K252a abrogated MOR axonal membrane integration, functional coupling, and agonist-elicited antinociception at sites of nerve injury. These findings suggest that local conceptual changes resulting from nerve injury are required for the establishment of functional axonal membrane MOR. Axonal integration and subsequent accessibility of functionally coupled MOR are of great relevance particularly for patients suffering from severe pain due to nerve injury or tumour infiltration.

Regulation of P2X Purinergic Receptor Signaling by Cholesterol

P2X receptors are cation-selective channels that are activated by the binding of extracellular ATP. They have a high permeability to Ca²⁺, Na⁺, and K⁺ and are expressed widely throughout the nervous, immune, cardiovascular, skeletal, gastrointestinal, respiratory, and endocrine systems. Seven mammalian subtypes of P2X receptor subunits have been identified, P2X1-7, and those that function as homotrimeric receptors (P2X1, 2, 3, 4, and 7) are targeted to lipid rafts, although they show limited resistance to solubilization by Triton X-100. Recent crystal structures of P2X3 and P2X4 receptors have provided considerable high-resolution information about the architecture of this family of receptors and yet the molecular details of how they are regulated by cholesterol are unknown. Currents mediated by the P2X1-4 receptors are either inhibited or relatively insensitive to cholesterol depletion, but there is no clear evidence to support the direct binding of cholesterol to these receptors. In contrast, the activity of the low-affinity, proinflammatory P2X7 receptor is potentiated by cholesterol depletion and regions within the proximal C-terminus play an important role in coupling changes in cholesterol to the gating of the pore. Based upon our understanding of the lipid signaling events that are triggered downstream of P2X7 receptor activation, a change in the levels of cholesterol may contribute to the sensitization of receptor currents and the dilation of the pore that occurs following prolonged, high-level stimulation. This chapter focuses on the regulation of P2X7 receptor signaling by cholesterol and our current understanding of the mechanisms that underlie this.

Annexin A6 controls neuronal membrane dynamics throughout chick cranial sensory gangliogenesis

Cranial sensory ganglia are components of the peripheral nervous system that possess a significant somatosensory role and include neurons within the trigeminal and epibranchial nerve bundles. Although it is well established that these ganglia arise from interactions between neural crest and neurogenic placode cells, the molecular basis of ganglia assembly is still poorly understood. Members of the Annexin protein superfamily play key roles in sensory nervous system development throughout metazoans. Annexin A6 is expressed in chick trigeminal and epibranchial placode cell-derived neuroblasts and neurons, but its function in cranial ganglia formation has not been elucidated. To this end, we interrogated the role of Annexin A6 using gene perturbation studies in the chick embryo. Our data reveal that placode cell-derived neuroblasts with reduced Annexin A6 levels ingress and migrate normally to the ganglionic anlage, where neural crest cell corridors correctly form around them. Strikingly, while Annexin A6-depleted placode cell-derived neurons still express mature neuronal markers, they fail to form two long processes, which are considered morphological features of mature neurons, and no longer innervate their designated targets due to the absence of this bipolar morphology. Moreover, overexpression of Annexin A6 causes some placode cell-derived neurons to form extra protrusions alongside these bipolar processes. These data demonstrate that the molecular program associated with neuronal maturation is distinct from that orchestrating changes in neuronal morphology, and, importantly, reveal Annexin A6 to be a key membrane scaffolding protein during sensory neuron membrane biogenesis. Collectively, our results provide novel insight into mechanisms underscoring morphological changes within placode cell-derived neurons that are essential for cranial gangliogenesis.

Single Cell Profiling Using Ionic Liquid Matrix-Enhanced Secondary Ion Mass Spectrometry for Neuronal Cell Type Differentiation

A high-throughput single cell profiling method has been developed for matrix-enhanced-secondary ion mass spectrometry (ME-SIMS) to investigate the lipid profiles of neuronal cells. Populations of cells are dispersed onto the substrate, their locations determined using optical microscopy, and the cell locations used to guide the acquisition of SIMS spectra from the cells. Up to 2,000 cells can be assayed in one experiment at a rate of 6 s per cell. Multiple saturated and unsaturated phosphatidylcholines (PCs) and their fragments are detected and verified with tandem mass spectrometry from individual cells when ionic liquids are employed as a matrix. Optically guided single cell profiling with ME-SIMS is suitable for a range of cell sizes, from Aplysia californica neurons larger than 75 μm to 7-μm rat cerebellar neurons. ME-SIMS analysis followed by t-distributed stochastic neighbor embedding of peaks in the lipid molecular mass range (m/z 700-850) distinguishes several cell types from the rat central nervous system, largely based on the relative proportions of four dominant lipids, PC(32:0), PC(34:1), PC(36:1), and PC(38:5). Furthermore, subpopulations within each cell type are tentatively classified consistent with their endogenous lipid ratios. The results illustrate the efficacy of a new approach to classify single cell populations and subpopulations using SIMS profiling of lipid and metabolite contents. These methods are broadly applicable for high throughput single cell chemical analyses.

Inefficient constitutive inhibition of P2X3 receptors by brain natriuretic peptide system contributes to sensitization of trigeminal sensory neurons in a genetic mouse model of familial hemiplegic migraine

BACKGROUND

On trigeminal ganglion neurons, pain-sensing P2X3 receptors are constitutively inhibited by brain natriuretic peptide via its natriuretic peptide receptor-A. This inhibition is associated with increased P2X3 serine phosphorylation and receptor redistribution to non-lipid raft membrane compartments. The natriuretic peptide receptor-A antagonist anantin reverses these effects. We studied whether P2X3 inhibition is dysfunctional in a genetic familial hemiplegic migraine type-1 model produced by introduction of the human pathogenic R192Q missense mutation into the mouse CACNA1A gene (knock-in phenotype). This model faithfully replicates several properties of familial hemiplegic migraine type-1, with gain-of-function of CaV2.1 Ca(2+) channels, raised levels of the algogenic peptide calcitonin gene-related peptide, and enhanced activity of P2X3 receptors in trigeminal ganglia.

RESULTS

In knock-in neurons, anantin did not affect P2X3 receptor activity, membrane distribution, or serine phosphorylation level, implying ineffective inhibition by the constitutive brain natriuretic peptide/natriuretic peptide receptor-A pathway. However, expression and functional properties of this pathway remained intact together with its ability to downregulate TRPV1 channels. Reversing the familial hemiplegic migraine type-1 phenotype with the CaV2.1-specific antagonist, ω-agatoxin IVA restored P2X3 activity to wild-type level and enabled the potentiating effects of anantin again. After blocking calcitonin gene-related peptide receptors, P2X3 receptors exhibited wild-type properties and were again potentiated by anantin.

CONCLUSIONS

P2X3 receptors on mouse trigeminal ganglion neurons are subjected to contrasting modulation by inhibitory brain natriuretic peptide and facilitatory calcitonin gene-related peptide that both operate via complex intracellular signaling. In the familial hemiplegic migraine type-1 migraine model, the action of calcitonin gene-related peptide appears to prevail over brain natriuretic peptide, thus suggesting that peripheral inhibition of P2X3 receptors becomes insufficient and contributes to trigeminal pain sensitization.

Brain natriuretic peptide constitutively downregulates P2X3 receptors by controlling their phosphorylation state and membrane localization

BACKGROUND

ATP-gated P2X3 receptors are important transducers of nociceptive stimuli and are almost exclusively expressed by sensory ganglion neurons. In mouse trigeminal ganglion (TG), P2X3 receptor function is unexpectedly enhanced by pharmacological block of natriuretic peptide receptor-A (NPR-A), outlining a potential inhibitory role of endogenous natriuretic peptides in nociception mediated by P2X3 receptors. Lack of change in P2X3 protein expression indicates a complex modulation whose mechanisms for downregulating P2X3 receptor function remain unclear.

RESULTS

To clarify this process in mouse TG cultures, we suppressed NPR-A signaling with either siRNA of the endogenous agonist BNP, or the NPR-A blocker anantin. Thus, we investigated changes in P2X3 receptor distribution in the lipid raft membrane compartment, their phosphorylation state, as well as their function with patch clamping. Delayed onset of P2X3 desensitization was one mechanism for the anantin-induced enhancement of P2X3 activity. Anantin application caused preferential P2X3 receptor redistribution to the lipid raft compartment and decreased P2X3 serine phosphorylation, two phenomena that were not interdependent. An inhibitor of cGMP-dependent protein kinase and siRNA-mediated knockdown of BNP mimicked the effect of anantin.

CONCLUSIONS

We demonstrated that in mouse trigeminal neurons endogenous BNP acts on NPR-A receptors to determine constitutive depression of P2X3 receptor function. Tonic inhibition of P2X3 receptor activity by BNP/NPR-A/PKG pathways occurs via two distinct mechanisms: P2X3 serine phosphorylation and receptor redistribution to non-raft membrane compartments. This novel mechanism of receptor control might be a target for future studies aiming at decreasing dysregulated P2X3 receptor activity in chronic pain.

Interictal, circulating sphingolipids in women with episodic migraine: A case-control study

OBJECTIVE

To evaluate interictal, circulating sphingolipids in women migraineurs.

METHODS

In the fasting state, serum samples were obtained pain-free from 88 women with episodic migraine (EM; n=52) and from controls (n=36). Sphingolipids were detected and quantified by high-performance liquid chromatography coupled with tandem mass spectrometry using multiple reaction monitoring. Multivariate logistic regression was used to examine the association between serum sphingolipids and EM odds. A recursive partitioning decision tree based on the serum concentrations of 10 sphingolipids was used to determine the presence or absence of EM in a subset of participants.

RESULTS

Total ceramide (EM 6,502.9 ng/mL vs controls 10,518.5 ng/mL; p<0.0001) and dihydroceramide (EM 39.3 ng/mL vs controls 63.1 ng/mL; p<0.0001) levels were decreased in those with EM as compared with controls. Using multivariate logistic regression, each SD increase in total ceramide (odds ratio [OR] 0.07; 95% confidence interval [CI]: 0.02, 0.22; p<0.001) and total dihydroceramide (OR 0.05; 95% CI: 0.01, 0.21; p<0.001) levels was associated with more than 92% reduced odds of migraine. Although crude sphingomyelin levels were not different in EM compared with controls, after adjustments, every SD increase in the sphingomyelin species C18:0 (OR 4.28; 95% CI: 1.87, 9.81; p=0.001) and C18:1 (OR 2.93; 95% CI: 1.55, 5.54; p=0.001) was associated with an increased odds of migraine. Recursive portioning models correctly classified 14 of 14 randomly selected participants as EM or control.

CONCLUSION

These results suggest that sphingolipid metabolism is altered in women with EM and that serum sphingolipid panels may have potential to differentiate EM presence or absence.

CLASSIFICATION OF EVIDENCE

This study provides Class III evidence that serum sphingolipid panels accurately distinguish women with migraine from women without migraine.

Effects of 5α-cholestan-3-one on the synaptic vesicle cycle at the mouse neuromuscular junction

We have investigated the effects of 5α-cholesten-3-one (5Ch3, 200 nM) on synaptic transmission in mouse diaphragm. 5Ch3 had no impact on the amplitude or frequency of miniature endplate currents (MEPCs, spontaneous secretion), but decreased the amplitude of EPCs (evoked secretion) triggered by single action potentials. Treatment with 5Ch3 increased the depression of EPC amplitude and slowed the unloading of the dye FM1-43 from synaptic vesicles (exocytosis rate) during high-frequency stimulation. The estimated recycling time of vesicles did not change, suggesting that the decline of synaptic efficiency was due to the reduction in the size of the population of vesicles involved in release. The effects of 5Ch3 on synaptic transmission may be related to changes in the phase properties of the membrane. We have found that 5Ch3 reduces the staining of synaptic regions with the B-subunit of cholera toxin (a marker of lipid rafts) and increases the fluorescence of 22-NBD-cholesterol, indicating a phase change within the membrane. Manipulations of membrane cholesterol (saturation or depletion) strongly reduced the influence of 5Ch3 on both FM1-43 dye unloading and staining with the B-subunit of cholera toxin. Thus, 5Ch3 reduces the number of vesicles which are actively recruited during synaptic transmission and alters membrane properties. These effects of 5Ch3 depend on membrane cholesterol.

Redox and nitric oxide-mediated regulation of sensory neuron ion channel function

SIGNIFICANCE

Reactive oxygen and nitrogen species (ROS and RNS, respectively) can intimately control neuronal excitability and synaptic strength by regulating the function of many ion channels. In peripheral sensory neurons, such regulation contributes towards the control of somatosensory processing; therefore, understanding the mechanisms of such regulation is necessary for the development of new therapeutic strategies and for the treatment of sensory dysfunctions, such as chronic pain.

RECENT ADVANCES

Tremendous progress in deciphering nitric oxide (NO) and ROS signaling in the nervous system has been made in recent decades. This includes the recognition of these molecules as important second messengers and the elucidation of their metabolic pathways and cellular targets. Mounting evidence suggests that these targets include many ion channels which can be directly or indirectly modulated by ROS and NO. However, the mechanisms specific to sensory neurons are still poorly understood. This review will therefore summarize recent findings that highlight the complex nature of the signaling pathways involved in redox/NO regulation of sensory neuron ion channels and excitability; references to redox mechanisms described in other neuron types will be made where necessary.

CRITICAL ISSUES

The complexity and interplay within the redox, NO, and other gasotransmitter modulation of protein function are still largely unresolved. Issues of specificity and intracellular localization of these signaling cascades will also be addressed.

FUTURE DIRECTIONS

Since our understanding of ROS and RNS signaling in sensory neurons is limited, there is a multitude of future directions; one of the most important issues for further study is the establishment of the exact roles that these signaling pathways play in pain processing and the translation of this understanding into new therapeutics.

Plasma membrane cholesterol as a regulator of human and rodent P2X7 receptor activation and sensitization

P2X7 receptors are nonselective cation channels gated by high extracellular ATP, but with sustained activation, receptor sensitization occurs, whereby the intrinsic pore dilates, making the cell permeable to large organic cations, which eventually leads to cell death. P2X7 receptors associate with cholesterol-rich lipid rafts, but it is unclear how this affects the properties of the receptor channel. Here we show that pore-forming properties of human and rodent P2X7 receptors are sensitive to perturbations of cholesterol levels. Acute depletion of cholesterol with 5 mm methyl-β-cyclodextrin (MCD) caused a substantial increase in the rate of agonist-evoked pore formation, as measured by the uptake of ethidium dye, whereas cholesterol loading inhibited this process. Patch clamp analysis of P2X7 receptor currents carried by Na⁺ and N-methyl-d-glucamine (NMDG⁺) showed enhanced activation and current facilitation following cholesterol depletion. This contrasts with the inhibitory effect of methyl-β-cyclodextrin reported for other P2X subtypes. Mutational analysis suggests the involvement of an N-terminal region and a proximal C-terminal region that comprises multiple cholesterol recognition amino acid consensus (CRAC) motifs, in the cholesterol sensitivity of channel gating. These results reveal cholesterol as a negative regulator of P2X7 receptor pore formation, protecting cells from P2X7-mediated cell death.

Chemical structure and morphology of dorsal root ganglion neurons from naive and inflamed mice

Fourier transform infrared spectromicroscopy provides label-free imaging to detect the spatial distribution of the characteristic functional groups in proteins, lipids, phosphates, and carbohydrates simultaneously in individual DRG neurons. We have identified ring-shaped distributions of lipid and/or carbohydrate enrichment in subpopulations of neurons which has never before been reported. These distributions are ring-shaped within the cytoplasm and are likely representative of the endoplasmic reticulum. The prevalence of chemical ring subtypes differs between large- and small-diameter neurons. Peripheral inflammation increased the relative lipid content specifically in small-diameter neurons, many of which are nociceptive. Because many small-diameter neurons express an ion channel involved in inflammatory pain, transient receptor potential ankyrin 1 (TRPA1), we asked whether this increase in lipid content occurs in TRPA1-deficient (knock-out) neurons. No statistically significant change in lipid content occurred in TRPA1-deficient neurons, indicating that the inflammation-mediated increase in lipid content is largely dependent on TRPA1. Because TRPA1 is known to mediate mechanical and cold sensitization that accompanies peripheral inflammation, our findings may have important implications for a potential role of lipids in inflammatory pain.

Ion channels and migraine

Migraine is one of the most common neurological disorders. Despite its prevalence, the basic physiology of the molecules and mechanisms that contribute to migraine headache is still poorly understood, making the discovery of more effective treatments extremely difficult. The consistent presence of head-specific pain during migraine suggests an important role for activation of the peripheral nociceptors localized to the head. Accordingly, this review will cover the current understanding of the biological mechanisms leading to episodic activation and sensitization of the trigeminovascular pain pathway, focusing on recent advances regarding activation and modulation of ion channels.

The therapeutic promise of ATP antagonism at P2X3 receptors in respiratory and urological disorders

A sensory role for ATP was proposed long before general acceptance of its extracellular role. ATP activates and sensitizes signal transmission at multiple sites along the sensory axis, across multiple synapses. P2X and P2Y receptors mediate ATP modulation of sensory pathways and participate in dysregulation, where ATP action directly on primary afferent neurons (PANs), linking receptive field to CNS, has received much attention. Many PANs, especially C-fibers, are activated by ATP, via P2X3-containing trimers. P2X3 knock-out mice and knock-down in rats led to reduced nocifensive activity and visceral reflexes, suggesting that antagonism may offer benefit in sensory disorders. Recently, drug-like P2X3 antagonists, active in a many inflammatory and visceral pain models, have emerged. Significantly, these compounds have no overt CNS action and are inactive versus acute nociception. Selectively targeting ATP sensitization of PANs may lead to therapies that block inappropriate chronic signals at their source, decreasing drivers of peripheral and central wind-up, yet leaving defensive nociceptive and brain functions unperturbed. This article reviews this evidence, focusing on how ATP sensitization of PANs in visceral "hollow" organs primes them to chronic discomfort, irritation and pain (symptoms) as well as exacerbated autonomic reflexes (signs), and how the use of isolated organ-nerve preparations has revealed this mechanism. Urinary and airways systems share many features: dependence on continuous afferent traffic to brainstem centers to coordinate efferent autonomic outflow; loss of descending inhibitory influence in functional and sensory disorders; dependence on ATP in mediating sensory responses to diverse mechanical and chemical stimuli; a mechanistically overlapping array of existing medicines for pathological conditions. These similarities may also play out in terms of future treatment of signs and symptoms, in the potential for benefit of P2X3 antagonists.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

ATP P2X3 receptors and neuronal sensitization

Increasing evidence indicates the importance of extracellular adenosine triphosphate (ATP) in the modulation of neuronal function. In particular, fine control of ATP release and the selective and discrete ATP receptor operation are crucial elements of the crosstalk between neuronal and non-neuronal cells in the peripheral and central nervous systems. In peripheral neurons, ATP signaling gives an important contribution to neuronal sensitization, especially that involved in neuropathic pain. Among other subtypes, P2X3 receptors expressed on sensory neurons are sensitive even to nanomolar concentrations of extracellular ATP, and therefore are important transducers of pain stimuli. P2X3 receptor function is highly sensitive to soluble factors like neuropeptides and neurotrophins, and is controlled by transduction mechanisms, protein-protein interactions and discrete membrane compartmentalization. More recent findings have demonstrated that P2X3 receptors interact with the synaptic scaffold protein calcium/calmodulin-dependent serine protein kinase (CASK) in a state dependent fashion, indicating that CASK plays a crucial role in the modulation of P2X3 receptor stability and efficiency. Activation of P2X3 receptors within CASK/P2X3 complex has important consequences for neuronal plasticity and possibly for the release of neuromodulators and neurotransmitters. Better understanding of the interactome machinery of P2X3 receptors and their integration with other receptors and channels on neuronal surface membranes, is proposed to be essential to unveil the process of neuronal sensitization and related, abnormal pain signaling.

Mutated CaV2.1 channels dysregulate CASK/P2X3 signaling in mouse trigeminal sensory neurons of R192Q Cacna1a knock-in mice

Background

ATP-gated P2X3 receptors of sensory ganglion neurons are important transducers of pain as they adapt their expression and function in response to acute and chronic nociceptive signals. The present study investigated the role of calcium/calmodulin-dependent serine protein kinase (CASK) in controlling P2X3 receptor expression and function in trigeminal ganglia from Cacna1a R192Q-mutated knock-in (KI) mice, a genetic model for familial hemiplegic migraine type-1.

Results

KI ganglion neurons showed more abundant CASK/P2X3 receptor complex at membrane level, a result that likely originated from gain-of-function effects of R192Q-mutated Ca_V2.1 channels and downstream enhanced CaMKII activity. The selective Ca_V2.1 channel blocker ω-Agatoxin IVA and the CaMKII inhibitor KN-93 were sufficient to return CASK/P2X3 co-expression to WT levels. After CASK silencing, P2X3 receptor expression was decreased in both WT and KI ganglia, supporting the role of CASK in P2X3 receptor stabilization. This process was functionally observed as reduced P2X3 receptor currents.

Conclusions

We propose that, in trigeminal sensory neurons, the CASK/P2X3 complex has a dynamic nature depending on intracellular calcium and related signaling, that are enhanced in a transgenic mouse model of genetic hemiplegic migraine.

The trafficking and targeting of P2X receptors

The functional expression of P2X receptors at the plasma membrane is dependent on their trafficking along secretory and endocytic pathways. There are seven P2X receptor subunits, and these differ in their subcellular distributions because they have very different trafficking properties. Some are retained within the endoplasmic reticulum (ER), while others are predominantly at the cell surface or within endosomes and lysosomes. Changes in recruitment of receptors to and from the plasma membrane provides a way of rapidly up- or down-regulating the cellular response to adenosine triphosphate (ATP). An additional layer of regulation is the targeting of these receptors within the membranes of each compartment, which affects their stability, function and the nature of the effector proteins with which they form signaling complexes. The trafficking and targeting of P2X receptors is regulated by their interactions with other proteins and with lipids and we can expect this to vary in a cell-type specific manner and in response to changes in the environment giving rise to differences in receptor activity and function.

Nerve growth factor/p75 neurotrophin receptor-mediated sensitization of rat sensory neurons depends on membrane cholesterol

Nerve growth factor (NGF) is an important mediator in the initiation of the inflammatory response and NGF via activation of the p75 neurotrophin receptor (p75(NTR)) and downstream sphingomyelin signaling leads to significant enhancement of the excitability of small-diameter sensory neurons. Because of the interaction between sphingomyelin and cholesterol in creating membrane liquid-ordered domains known as membrane or lipid rafts, we examined whether neuronal NGF-induced sensitization via p75(NTR) was dependent on the integrity of membrane rafts. Here, we demonstrate that the capacity of NGF to enhance the excitability of sensory neurons may result from the interaction of p75(NTR) with its downstream signaling partner(s) in membrane rafts. Two agents known to disrupt membrane rafts, edelfosine and methyl-β-cyclodextrin (MβCD), block the increase in excitability produced by NGF. In contrast, treatment with MβCD containing saturated amounts of cholesterol does not alter the capacity of NGF to augment excitability. In addition, adding back MβCD with cholesterol restored the NGF-induced sensitization in previously cholesterol-depleted neurons, suggesting that cholesterol and the structural integrity of rafts are key to promoting NGF-mediated sensitization. Using established protocols to isolate detergent-resistant membranes, both p75(NTR) and the neuronal membrane raft marker, flotillin, localize to raft fractions. These results suggest that downstream signaling partners interacting with p75(NTR) in sensory neurons are associated with membrane raft signaling platforms.

P2Y purinergic receptors: new targets for analgesic and antimigraine drugs

Millions of individuals worldwide suffer from acute and, more severely, chronic pain conditions (e.g., neuropathic pain, and migraine). The latter bear tremendous personal, familial, and social costs, since sufferers and their relatives undergo a complete turnaround of their lives with the search of relief from pain becoming their pivotal thought. Sadly, to date no effective pharmacological approaches are available which can alleviate chronic pain significantly or in the long run in all patients. The current central strategy for the development of new and effective painkillers lies in the hypothesis that cellular and/or molecular players in nociception must exists that are not targeted by "classical" analgesics, and therefore researchers have put tremendous efforts into the in-depth analysis of the pathways leading to pain development and maintenance over time. In this complex scenario, two outsiders are now taking the center stage: glial cells in sensory ganglia and in the central nervous system, thanks to their ability to communicate with neurons and to modulate their firing, and the purinergic system. Extracellular purine and pyrimidine nucleotides are involved in the physiology of virtually every body district, and their extracellular concentrations massively increase under pathological situations, suggesting that they might represent potential targets for the modulation of disease-associated symptoms, like pain. Here, we provide an overview of the present knowledge of the role of nucleotides in nociception, with a particular emphasis on G protein-coupled P2Y receptors and their involvement in the communication between first- and second-order neurons in sensory nerve pathways and surrounding glial cells.